Fix overlapping typespec

HTTPoison already has a typespec for process_request_headers

    lib/forcex/api/http.ex:8:overlapping_contract
    Overloaded contract for Forcex.Api.Http.process_request_headers/1 has
    overlapping domains; such contracts are currently unsupported and
    are simply ignored.

lib/forcex/api/http.ex

@@ -46,7 +46,6 @@ defmodule Forcex.Api.Http do
   defp process_response(%HTTPoison.Response{body: body, status_code: 200}), do: body
   defp process_response(%HTTPoison.Response{body: body, status_code: status}), do: {status, body}
 
-  @spec process_request_headers(list({String.t, String.t})) :: list({String.t, String.t})
   def process_request_headers(headers), do: headers ++ @user_agent ++ @accept ++ @accept_encoding
 
   @spec process_headers(list({String.t, String.t})) :: map